The synchronization of transmitters and receivers is a central part of modern digital communications systems. In wireless digital communications systems, for instance time multiplex systems (TDMA) or code multiplex systems (CDMA), of which the code multiplex system has the two most usual solutions in frequency hopp systems (FHSS) and direct sequence systems (DSSS), it is necessary to mutually synchronize transmitters and receivers so that the receiver will receive the correct time slot in TDMA or the correct code phase in CDMA.
One method of synchronizing transmitters and receivers in digital communications systems is for the transmitter to transmit a digital sequence that is known to the receiver. The receiver lies in a search procedure in which the receiver searches for the known digital sequence. When a digital sequence known to the receiver is found, a sync. time pulse is generated and used as a time reference, therewith synchronizing transmitter and receiver.
The ability to receive a system identification signal is also an application of asynchronous reception. A search procedure in which the receiver searches for known signals is also applied in automatic identification systems. One such automatic identification system is, e.g., Radio Frequency Identification (RFID) with which the location of, e.g., vehicles, employees, criminals and animals can be monitored.
The object to be monitored carries a transmitter which transmits a unique signal. This signal is transmitted at a point in time unknown to the receiver, meaning that the signal is asynchronous and this is registered in the signal upon detection of the known signal.
A well-known technique for the asynchronous reception of the digital sequence known to the receiver involves the use of a correlator which, e.g., can be implemented as a transversal filter where the length of the filter is equal to the length of the known digital sequence. This filter effects correlation of a received digital sequence and the digital sequence earlier known to the receiver, resulting in a value which is proportional to the similarity between the received sequence and the known sequence. In order for a received known digital sequence to be registered, the result of the correlation must exceed a predetermined threshold value.
The performance that can be achieved through the aforedescribed correlation is directly dependent on the length of the known digital sequence. The longer a known digital sequence, the better the performance achieved with the correlation. However, there is a practical upper limit to the length of the transversal filter as long filters result in high power consumption. It is important to keep power consumption at a low level in, e.g., mobile units. Long transversal filters are also complex in implementation.
A drawback with the aforedescribed technique is thus that a long filter results in high power consumption and that its implementation is also complex.
Another drawback with the use of long transversal filters is that their implementation requires a large memory area; the memory area is limited in mobile units.
Still another drawback with the aforedescribed technique is that the length of the known sequence is permanent, i.e. cannot be changed.
U.S. Pat. No. 5,422,916 describes a synchronizing method using a known digital sequence where the ambient surroundings may have influenced the received sequence with a burst of incident noise, such that identification of a known digital sequence requires more than just correlating the received digital sequence and the known digital sequence. A 64-bit sequence derived from a so-called Barker sequence is used as the known digital sequence.
This known method involves comparing the received digital sequence with the known digital sequence so as to count the number of errors in the received digital sequence. If the result exceeds a determined threshold value, the detection process is continued by checking that the number of errors in the received digital sequence does not exceed an upper limit. If such is not the case, the received digital sequence is divided into four parts each consisting of 16 bits. These four parts are linked together two and two, resulting in six new 32-bit words. The number of errors is then calculated in each of these new 32-bit words, and a counter is advanced by one increment for each of the words in which the number of errors does not exceed a specific value. Subsequent to having checked all six words, it is assumed that the known digital sequence has been received when the result in the counter exceeds a specific value.
The known method solely solves the problem concerned with bursts of incident noise, but the problem associated with long correlators and long correlations remains.
In the PIMRC conference held in September 1995, a report was published on a hybrid parallel correlator (An Improved Hybrid PN Code Acquisition for CDMA Personal Wireless Communication, IEEE-95:0-7803-3002-1/95). The hybrid parallel correlator is described in this document, i.e. a mixture of a serial and parallel correlator. The known sequence is divided into segments that are contingent on two construction parameters N.sub.1 and N.sub.2. These parameters are chosen differently with respect to the desired degree of parallelism (N.sub.1) and serialism (N.sub.2). The code access time is low when many parallel correlators are used, whereas machine hardware complexity becomes high. The hardware becomes simple when a serial correlator is used, whereas the code access time is long. The method described in said document represents a compromise between parallel and serial correlator. The length M of the segment is chosen in accordance with M=.theta./(N.sub.1 .times.N.sub.2), where .theta. is the length of the known sequence. Each of the correlators includes one of the M-segments as a correlation segment. When a first segment is found, the system switches from a search mode (H.sub.0) to a verification mode (H.sub.1). A-tests are carried out in the verification mode, and if B-tests thereof have a correlator output signal which exceeds a set of threshold values, a switch is made to a tracing process. The access process is terminated when the correct code phase is delivered to the code tracing system, otherwise the access process is reactivated when a false code phase is delivered.
The method relates solely to a manner of increasing correlation reception rates and can be applied in systems in which this is critical. The method does not therefore solve the problems which the present invention intends to solve.